The direct oxidation of ethylene to ethylene oxide is practiced commercially using supported silver catalysts. Unfortunately, the analogous direct oxidation of olefins containing allylic hydrogens such as propylene generally exhibits significantly lower selectivity to the epoxide due to side reactions. In view of the problems encountered with supported silver catalysts of the type used for ethylene oxidation, alternative catalyst systems have been explored for use with higher olefins in recent years.
For example, U.S. Pat. No. 5,623,090 describes the production of an epoxide from an unsaturated hydrocarbon by passing a mixture comprised of molecular hydrogen, the unsaturated hydrocarbon and oxygen through a bed of a catalyst comprising a titanium dioxide carrier and ultrafine gold particles deposited on the carrier. According to the patent, "it is essential to use gold and titanium dioxide in combination." The inventors named in the patent have similarly reported (Hayashi et al., Symposium on Heterogeneous Hydrocarbon Oxidation, Presented before the Division of Petroleum Chemistry, Inc., 211th National Meeting, American Chemical Society, New Orleans, La, Mar. 24-29, 1996) that the use of gold supported on metal oxides other than TiO.sub.2 does not lead to the partial oxidation of propylene. Other publications related to supported gold catalysts have also emphasized the criticality of having titanium present in the support in order to attain an active olefin epoxidation catalyst. See, for example, WO 97/34692, WO 98/00413, WO 98/00414, and WO/00415.